Graph tooltips

When the user clicks on one of the barcharts, we now display a tooltip displaying the info for that particular day. I.e
![screen shot 2014-12-09 at 10 11 39 pm](https://cloud.githubusercontent.com/assets/1654201/5354700/670fe002-7ff0-11e4-93e3-6bd196ab11ce.png)

This is done via a d3 extension called d3-tips.